The present invention relate to a flat-panel display device wherein a plurality of display pixels are arranged in a matrix to display an image, and more particularly, to a flat-panel display device wherein a driving circuit is integrated on a substrate together with switching elements of the display pixels.
A liquid crystal display device is a flat-panel display device having characteristics of being thin and light-weight, and low power consumption, and is widely used in various fields such as television receivers and office automation devices because of the characteristics. For example, in an active-matrix type liquid crystal display device, a plurality of pixel electrodes, switching elements, scanning lines and signal lines are formed on a transparent glass plate. These pixel electrodes are arranged in a matrix, and a plurality of switching elements are respectively arranged to be adjacent to the pixel electrodes. A plurality of scanning lines are arranged along columns of the pixel electrodes, and a plurality of signal lines are arranged along the rows of the pixel electrodes. Each switching element becomes conductive upon driving of a corresponding scanning line to apply the potential of a corresponding signal line to a corresponding pixel electrode.
Recently, as a measure to produce such a liquid crystal display device at a low cost, it is considered to integrate a driver circuit such as a scanning driver for driving the scanning lines and a signal line driver for driving the signal lines, together with the switching elements, on the glass plate. Specifically, a plurality of thin film transistors are formed in a common manufacturing process as the switching elements, the scanning line driver, and the signal line driver. The signal line driver is constituted by, for example, a shift register and a plurality of analog switches. The shift register determines the sampling timings of a video signal supplied from the outside, and the analog switches sequentially sample the video signal under the control of the shift register to supply results of sampling to the respective signal lines.
Since the thin film transistor is formed by use of a non-monocrystalline semiconductor layer, it cannot easily obtain preferable operation characteristics therefrom and limits the sampling rate and the current driving ability of the signal line driver. This makes it difficult to sequentially sample the video signal with the adequate time margin. It is considered that such a problem can be solved by taking advantage of a video signal bus having a plurality of transmission lines. For example, when the video signal bus has two transmission lines through which odd-column and even-column video signals derived in advance from the video signal are transmitted in parallel to the signal line driver, the first transmission line is connected to odd-numbered signal lines via half of the analog switches, and the second transmission line is connected to even-numbered signal lines via the remaining half of the analog switches. The shift register is connected to sequentially drive groups of analog switches each assigned to corresponding two adjacent signal lines. The analog switches of each group simultaneously sample the odd-column video signal and the even-column video signal under the control of the shift register and supply them to the corresponding two adjacent signal lines, and therefore, the time margin of the sampling operation can be improved.
However, the wiring connecting the first transmission line with half of the analog switches intersects the wiring connecting the second transmission line and the remaining half of the analog switches at many portions, thereby creating a parasitic capacitance corresponding to the capacitive coupling between the wirings. Since this parasitic capacitance narrows the band width of the video signals to be transmitted, a problem that an excellent image cannot be displayed thereby arises. Further, the influence of the parasitic capacitance may become further serious when the number of pixels are increased to obtain a larger screen size or higher resolution in the liquid crystal display device.